The present invention relates to a biaxially stretch-oriented film suitable for the process of transfer metallizing. More particularly, the present invention relates to films comprising a base layer which is coextruded with a covering layer on either side thereof and which comprises an inert admixture and an additive capable of migrating into the covering layers. Such films are disclosed, for example, in EP-A-0 175 259 and EP-A-0 216 342. These known films contain inert admixtures, but these are only present in the covering layers, in very small quantities. The admixtures serve to improve the slip and/or blocking resistance of the film.
The packaging industry requires great quantities of metallized papers and cardboard articles. The metal layer ensures good barrier properties to prevent flavor, grease, moisture and atmospheric oxygen from escaping or penetrating, respectively, and it gives the products a very attractive appearance. Paper provided with a metal layer is thus excellently suitable for use as a packaging material for tobacco products, ice cream, chocolates, sweets, chewing gum or for fatty foodstuffs, such as butter or cheese. Metallized paper is further used for the packaging of cosmetic products or of sterile articles for medical applications. Another field of use is the production of labels. In these applications, the metal layer is usually printed.
For the production of the paper-metal composite materials the process of transfer metallizing has proved successful in recent years. In a transfer process, the metal layer is transferred from a vacuum metallized polypropylene film to the paper. The following individual steps are performed in this process: Continuously metallizing the film web by vapor-depositing a metal layer; coating the metal layer applied to the film or the paper with an adhesive; laminating to produce a composite structure comprising film-metal-adhesive-paper; curing the adhesive; delaminating the composite structure, i.e., separating it at the metal-film interface and peeling off the film. The film used as the metal transfer medium is then wound up and is thus ready for another metallizing operation. Using appropriate formulations it is possible to metallize the film more than 40 times.
The adhesive is normally cured during storage of the laminated composite structure within a couple of days or also "in-line" by electron beam curing, i.e., by irradiating the composite structure with high-energy electrons. Curing of the adhesive by means of electron beams has gained increasing importance in the transfer metallizing process (cf., for example, "Export Polygraph International", No. 2, page 108 (1986).
The paper obtained by means of transfer metallizing has a very smooth, mirror-like, high-gloss metallic surface, and because of this decorative characteristic it is frequently used for the production of high-quality gift paper, greeting cards and packages, in which the metal layer is printed.
In other fields of application, in contrast, the metal layer of the paper-metal composite should have a matte finish or a textured surface or surface areas with varying degrees of gloss should be produced.
It has now been found that the polypropylene films which have hitherto been employed for transfer metallizing are less suitable for use in the process, if the adhesive applied is hardened by the action of electron beams, because the mechanical properties of the film are impaired by the electron beams. As a result, the rate of reuse, i.e., the possibility of metallizing the film repeatedly, is reduced to such an extent that the process can no longer be carried out in an economical way.